In many wireless networks, such as long-term evolution (LTE) networks, the selection of shared data channels for uplink transmission is scheduling/grant-based, and the scheduling and grant mechanisms are controlled by a base station (BS) in the network. A user equipment (UE) sends an uplink scheduling request to the BS. When the BS receives the scheduling request, the BS sends an uplink grant to the UE indicating its dedicated uplink resource allocation. The UE then transmits data on the granted resource. This is a specific example of grant-based uplink traffic. eMBB (enhanced mobile broadband) traffic is a specific example of grant-based traffic.
In contrast, for grant-free traffic, independent of any scheduling request, a UE may be mapped to resources for grant-free transmission. The resources are not dedicated to a UE, and there can be multiple UEs mapped to the same resource. In grant-free transmission, a UE does not rely on a dynamic scheduling request and grant mechanism to transmit data. A given UE that has no data to transmit will not transmit using the mapped resources.
Some grant-free traffic requires high reliability (for example error rates on the order of ˜1×10−5) and low latency (for example, 0.5 ms each way). URLL (ultra-reliable low latency) traffic may employ grant-free communication.
Grant-free traffic typically contains short packets and is mostly sporadic in nature. Dedicated resource assignment to grant-free UEs can render a system very inefficient, since unused radio resources results in lost capacity (assuming pending grant-based traffic). Improved ways to efficiently use those resources are desired.